1. Field of the Invention
This invention relates to improvements in transistor circuits, and more particularly, to improvements in circuits for reducing the turnoff time of power transistors, and still more particularly to improvements in circuitry for reducing the turnoff time of field effect transistors used for driving inductive loads, stator windings in direct current motors, and the like.
2. Description of the Relevant Art
The problem addressed by this invention is encountered especially in integrated power drivers when used as current sources for inductive loads such as stator windings in direct current motors. Typically, the transistor types used for power driving are either bipolar NPN's or N-channel MOSFET's. When driving an inductive load, a recirculation or free wheeling diode may be provided to supply a path for the load current at turnoff.
Thus with reference to FIG. 1, one prior art circuit embodiment typically may include an n-channel MOSFET power transistor 10 that provides drive current to an inductive load 12 and non-inductive circuitry, indicated generally as box 14. Typically inductive load 12 is a stator winding in a dc motor such as those used in floppy, winchester and compact disk drives. A diode 16 is connected in parallel with inductive load 12 to provide a recirculation path for the current in the load 12 when transistor 10 is turned off. Thus, when the current to inductive load 12 is turned off, the inductive action of the coil 12 will tend to maintain the current flowing from the source connection through the load 12 to ground. This continued current is conducted by freewheeling diode 16.
To turn off power transistor 10, its gate is switched from a positive voltage Vp to ground by switch 21 as indicated by signal 18. Switch 21 represents the commutation control circuitry of a disk drive. Turning off the power transistor 10 may, in fact, be difficult, due to the fact that inductive load 12 pulls the source of power transistor 10 below ground. Because its source is pulled negative, even with its gate at ground potential, the power transistor 10 remains on despite the transistor 10's gate being driven low.
With reference now to prior art FIG. 2, a circuit similar to the prior art circuit of FIG. 1 is shown but has an additional transistor 20 and a switch 22. The drain of transistor 20 is connected to the gate of power transistor 10 and the source of transistor 20 is connected to the drain of power transistor 10. Transistor 20 has a parasitic or intrinsic diode 23 which effectively has a cathode connected to its drain and an anode connected to its source.
In operation, power transistor 10 is turned off by switching switch 21 from Vp to ground or an open circuit while simultaneously switching switch 22 from ground to Vp, as shown by signal 18 and 24 in FIG. 2. Switch 21 and 22 represent the commutation control circuitry of the disk drive. As power transistor 10 turns off, its source is pulled low due to the inductive action of the winding 12 which accelerates the depletion of charge from the gate of power transistor 10 through the conductive path of transistor 20. Transistor 20 in this circuit effectively shortens the turn-off time of power transistor 10 by facilitating the depletion of the charge from the gate of power transistor 10. Additionally, power transistor 10 is held off, even if the inductive voltage pulls the source below ground by effectively "clamping" the gate and source of power transistor 10 at about the same voltage.
If switch 21 is grounded instead of floating, then the charge is quickly depleted from the gate of power transistor 10 which further facilitates its turn-off. However, large currents are drawn from ground and through transistor 20 when the source of transistor 10 is pulled low by the recirculation of the inductor. Therefore, it is the designers choice whether to turn off the power transistor by switching its gate to ground or by clamping its gate to source or by both.
It has been observed that, in some applications, the V.sub.gs (gate to source voltage) across transistor 20 can be very high due to the fact that transistor 20 requires significant positive gate voltages, relative to the source, to turn on and since the source voltage can be driven to large negative voltages by the recirculation of inductive load 12. This potentially large V.sub.gs may cause reliability problems in transistor 20.
It has also been observed that the voltage at the source of transistor 20 is fed back to the gate of power transistor 10 through the parasitic diode 23 of transistor 20, even this feedback path through the parasitic diode 23 may have undesirable effects to the stability and response of the circuit.